Salutogenic Effects of the Environment: Review of Health Protective Effects of Nature ... · 2017. 9. 16. · Salutogenic Effects of the Environment: Review of Health Protective Effects

Salutogenic Effects of the Environment: Review ofHealth Protective Effects of Nature and Daylight

Femke Beute* and Yvonne A.W. de KortEindhoven University of Technology, The Netherlands

Both nature and daylight have been found to positively influence health. Thesefindings were, however, found in two separate research domains. This paperpresents an overview of effects found for daylight and nature on health and thehealth-related concepts stress, mood, and executive functioning and self-regulation. Because of the overlap in effects found and the co-occurrence ofboth phenomena, the paper points to the need to consider daylight factors wheninvestigating effects of nature and vice versa. Furthermore, the existence ofpossibly shared underlying mechanisms is discussed and the need to unify theresearch paradigms and dependent variables used between the two researchfields. Last, in view of the beneficial effects of both phenomena on health, ourobjective is to raise awareness amongst the general public, designers, and healthpractitioners to use these naturally available phenomena to their full potential.

Keywords: daylight, health, nature, restoration, salutogenesis

Can this earth be so green and wonderful!And the sky shine so blue and clear!We stand and marvel. It’s hard to believe that this world can truly be ours . . .(H.M. Vesaas)

INTRODUCTION

Environmental factors influence our health in many ways, both positivelyand negatively. Noise and density are examples of factors that can affectour health negatively; other factors, such as a view of nature, are seen ascontaining health protective qualities. In this paper we will discuss two envi-ronmental factors that have been found to exhibit these health protectivequalities—daylight and nature.

* Address for correspondence: Femke Beute, Human Technology Interaction, EindhovenUniversity of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. Email:[email protected]

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APPLIED PSYCHOLOGY: HEALTH AND WELL-BEING, 2014, 6 (1), 67–95doi:10.1111/aphw.12019

© 2013 The International Association of Applied Psychology

Beneficial effects of natural environments have been found for stress,mental fatigue, and self-regulation failure. We will closely inspect howviewing nature can help overcome these very prominent threats to mental andphysical health in our present-day society. In addition, we will point toliterature which indicates that exposure to daylight can have very similarbeneficial effects. We will present an overview of the proposed pathwaysthrough which both factors can influence health and will critically assess theempirical evidence for their beneficial effects.

Our aim in writing this paper was to examine the range of health outcomesthat have been related to exposure to both nature and daylight, and toexplore and systemise the various underlying mechanisms of beneficial effectsthat have been suggested in the literature. To this end we have performed anextensive search of relevant publications in scientific domains includingmedical and psychiatric sciences, epidemiology, clinical, social, environmen-tal and health psychology, architecture and landscape architecture, targetingstudies that report effects of exposure to nature or daylight. Our aim was tocover the most important classes of health effects and mechanisms. Thispaper does not present a meta-analysis of effects of daylight and nature onhealth, nor does it present a systematic review according to the Cochranemethod; hence it cannot claim to be comprehensive on the level of individualstudies. We did not employ rigid inclusion criteria with regard to researchdesigns as this would inevitably have resulted in omitting crucial papersbecause of the plurality in research traditions between the different researchdomains. In bringing together empirical findings on health outcomes fromthese different research domains, we reveal the overlap in beneficial healtheffects of both phenomena.

We focused mainly on effects of passively viewing either real or mediatednature (slideshows and videos). There is also a large body of research lookingat active nature participation as wilderness experience and horticulturetherapy. We only included studies investigating effects of actively interactingwith nature when a control condition also involving physical activity wasincluded, because physical exercise can be beneficial for health on its own(Thompson Coon et al., 2011). In considering effects of daylight we will alsoreport on studies using electric bright white light, as long as the studies wereconducted during the biological day. There is a large body of evidence sug-gesting melatonin suppressing and alerting effects of bright light exposureduring the biological night. As we are only interested in effects of daytimelight, we will not report on these effects (for an overview, see e.g. Boyce,Hunter, & Howlett, 2003; CIE, 2009).

Even though exposure to nature and daylight often coincide, whetheroutdoors or indoors through windows, in the scientific literature both phe-nomena are almost exclusively studied separately from each other. One dis-advantage of this detachment is that in some studies restorative outcomes of

68 BEUTE AND DE KORT


one phenomenon potentially confound the other. The present paper urgesscholars to critically reassess their previous findings for possible confounds ormotivate them to reconsider the design of future studies. More importantly,however, we hope to learn more about potential underlying mechanisms andways to explore those further. Below, we will sequentially discuss effects ofnature and daylight on stress, mood, and executive functioning and self-regulation as important elements of health, and lastly discuss reported effectson health. But first, we take a look at the proposed underlying pathways forbeneficial effects of nature and daylight.

PATHWAYS DESCRIBED IN THE LITERATURE

The effects of nature on health are attributed to underlying psychologicalpathways. Psycho-evolutionary theory argues that evolution has favoredthose individuals who had an adaptive response to unthreatening naturalenvironments that contained elements beneficial for well-being (stress reduc-tion and restoring energy) as well as for survival (i.e. low risk, food and wateravailable; Ulrich et al., 1991). According to Ulrich and colleagues (1991), apre-cognitive emotional response exists which subsequently influences atten-tion, physiological responding, and behavior. Viewing unthreatening natureshould therefore help people become restored from stress by reducing nega-tive affect, increasing positive affect, and decreasing physiological arousal(Ulrich et al., 1991). The biopsychosocial model of challenge and threat(Blascovich & Mendes, 2000) proposes that person–environment interactionsconsist of affective, cognitive, and physiological responses. Indeed, researchinto the effects of nature has found effects in all three components, as we willsee in later sections.

Attention Restoration Theory (ART; S. Kaplan, 1995) was proposed toexplain beneficial effects of nature on executive functioning. Stephen Kaplan(1995) argues that certain activities and environments, including modern life inurban environments, can cause a phenomenon labeled attention fatigue. ARTdistinguishes between two types of attention: voluntary and involuntary.Voluntary attention, also called directed attention, is under conscious control,and prolonged exertion of this control requires effort. Over time, the resourcenecessary to control attention will become depleted resulting in directedattention fatigue. This state is characterised by fatigue, an inability to concen-trate, and irritability (S. Kaplan, 1995). The conceptualisation of directedattention as a limited resource is quite similar to the notion of self-regulatorycapacity—ego-strength (Baumeister, 1998)—as a limited resource and mayeven describe the same phenomenon (Kaplan & Berman, 2010).

ART states that it is through the mechanism of involuntary attention thatthe depleted directed attention resource can become replenished (Berman,Jonides, & Kaplan, 2008; Kaplan & Kaplan, 1982). Involuntary attention is

HEALTH PROTECTIVE EFFECTS OF NATURE AND DAYLIGHT 69


a bottom-up controlled process (Berman et al., 2008) that needs no consciouscontrol or inhibition, because attention is drawn immediately by fascinatingstimuli. It has further been postulated that soft fascination, in particular,makes stimuli restorative (S. Kaplan, 1995). Softly fascinating stimuli drawattention while at the same time leaving room for contemplation. Hardfascination, on the other hand, also draws attention automatically but in anall-consuming fashion. An example of hard fascination is watching engagingtelevision content. Soft fascination such as that elicited by nature enablesrestoration from directed attention fatigue.

Whereas proposed underlying mechanisms for the restorative effects ofnature are considered mostly psychological, beneficial health effects of day-light are mainly attributed to biological processes. There are at least twoknown biological pathways through which daylight can positively influencehealth. First of all, vitamin D is produced when sunlight touches our skin,which in turn has been linked to many health outcomes, such as reduced risksof cancer and cardiovascular disease (Kauffman, 2009), but also to improvedmood via the production of serotonin (Landsdowne & Provost, 1998).

A second pathway for daylight to affect health is through the circadiansystem. Retinal ganglion cells with a direct connection to the suprachiasmaticnucleus (SCN) in which our biological clock resides are particularly sensitiveto light with wavelengths within the blue spectrum (Brainard et al., 2001).Daylight naturally contains light across the full visual spectrum, includingblue light. In this way, daylight acts as an environmental cue to correctlyentrain our circadian rhythm to the night-dark cycle. Moreover, bright lightinduces direct responses in alertness-related subcortical structures (hypo-thalamus, brainstem, thalamus) limbic areas (amygdale and hippocampus),and even in cortical areas (Vandewalle, Maquet, & Dijk, 2009). Furthermore,effects on brain serotonin turnover have been proposed to run through retinallight exposure (aan het Rot, Benkelfat, Boivin, & Young, 2008a). This impliesthat, in addition to entraining the biological clock, light shows immediateeffects on alertness, cognitive performance, and even affective responses(Vandewalle et al., 2009).

These first pathways for the effects of daylight on health pertain to purelybiological mechanisms, but psychological mechanisms have also been pro-posed (e.g. Boyce et al., 2003). It is therefore possible that—much likenature—daylight positively influences well-being through affective, associa-tive, or appreciative routes.

Having introduced the proposed underlying pathways, we will now look atthe evidence for salutogenic effects of nature and daylight on stress, mood,executive functioning and self-regulation, and mental and physical health. Inthe supplementary materials, we present overview tables of all individualstudies with study type (broadly categorised as cross-sectional, quasi-experimental, and experimental designs), sample size, intervention type, and



study outcome. Furthermore, we report whether and how possible confoundswere controlled in each study.

STRESS

We all experience stress from time to time. Stress influences our mental state,inducing a shift towards more negatively toned emotions (Thayer &Brosschot, 2005; Ulrich et al., 1991), as well as our physiological status—often labeled the fight-or-flight response (Selye, 1950). This fight-or-flightresponse has evolutionary significance by enhancing quick physical responsesto dangerous situations, for instance by enabling quick transport of oxygenand nutrients to the muscles. Importantly, this physiological response is alsoactivated for stressful situations in which no physical response is required.Moreover, we turn on this response when we are ruminating over a stressorin the past or when we are anticipating a stressor in the future, which willresult in prolonged physiological activation (Brosschot, Pieper, & Thayer,2005). It is exactly this prolonged activation of the autonomic nervous systemthat can be harmful for health. More specifically, it is the accumulating effectof activating the stress-response (“allostatic load”; McEwen, 1998) that canincrease the risk for certain diseases (e.g. cardiovascular disease, arthritis,diabetes) as well as worsen already existing illnesses (Cohen et al., 2012).Prolonged stress can even cause damage to the hippocampus, degradingmemory (Sauro, Jorgensen, & Pedlow, 2003). Last, high levels of arousal andan increase in negative emotions can influence cognitive performance directlyas well (Ellenbogen, Schwartzman, Stewart, & Walker, 2002).

Some claim that it is mostly a lack of restoration that causes stress to affecthealth negatively (Brosschot et al., 2005). Several ways are proposed torecover from stress, such as for instance engaging in social interaction, exer-cise, or meditation. We will now discuss how nature and daylight can helprecover from stress.

Nature and Stress ReductionA recent fMRI study demonstrated that living in urban areas changes brainresponses to stress in a negative way compared to dwelling in a small town orrural area (Lederbogen et al., 2011). This study indicated that stress induc-tion produced higher brain activity in brain areas related to stress and nega-tive affect among people who grew up in a highly urbanised area. Not allstudies provide insight on whether the negative effects of cities are due to alack of nature or the presence of other urban factors and stressors. There are,however, also urban studies demonstrating restorative effects of nature. Anexploratory study in a deprived area of Scotland revealed that more green inthe neighborhood was related to a steeper cortisol slope (indicating a more



healthy cortisol secretion system), together with lower levels of self-reportedstress (Ward Thompson et al., 2012).

The stress-reducing potential of nature was reported by, among others,Ulrich and colleagues (1991) who had participants watch a stressful moviefollowed by one of six movies recorded in either a natural or an urbanenvironment. They found that physiological recovery from the stressfulmovie was faster after viewing nature movies than after urban movies.Furthermore, positive affect increased more, and negative affect decreasedmore, after watching movies of natural scenes than after watching movies ofurban scenes. Other studies have also found that watching nature can lowerphysiological arousal (Laumann, Gärling, & Stormark, 2003) and increasemood (Berman et al., 2008; Hartig, Evans, Jamner, Davis, & Gärling, 2003).In a study by Laumann and colleagues (2003), cardiac inter-beat interval wascontinuously monitored. They found that heart rate decreased for partici-pants viewing a natural video after performing a task inducing mental fatigueas compared to their baseline, while for participants watching an urban videoheart rate remained constant. Hartig and colleagues (2003) demonstratedthat walking in a natural as opposed to an urban environment resulted in adecrease in blood pressure. Evidence for the stress-reducing effect of viewingnatural environments was also found in a study not specifically designed totest psycho-evolutionary theory. Fredrickson and Levenson (1998) studiedthe effects of positive affect on recovery of the cardiovascular system follow-ing an emotional stressor. They found that cardiovascular recovery was fasterwhen viewing movies with positive content compared to watching either aneutral or sad movie (abstract movie of sticks and sad movie scene in whicha boy watches his father die, respectively). Importantly, both positive moviesconsisted of natural content: the first one showed waves breaking on a beach,the other portrayed a playful puppy.

There is also evidence suggesting an immunising effect of viewing naturalscenes on stress (Parsons, Tassinary, Ulrich, Hebl, & Grossman-Alexander,1998). In this study, participants viewed a video of a simulated drive justbefore and immediately after a stressor. The content of the simulated driveswas either natural or urban. Not only did they find that stress recovery wasfaster when viewing the natural scenes, the findings also suggested that thepre-stressor simulated drive through a natural area decreased the intensity ofthe stress response to the stressor. This buffering effect was, however, onlyfound on one of the four indicators (i.e. on skin conductance, not on cardio-vascular measures, facial EMG, or performance).

Daylight and Stress ReductionThere are only indirect indications that light too has stress-reducing effects.Exposure to bright electric light has been found to improve heart rate



variability in healthy subjects (Rechlin, Weis, Schneider, Zimmermann, &Kaschka, 1995). Increased heart rate variability has been related to increasedactivation of the parasympathetic nervous system (Bilchick & Berger, 2006),indicating potential stress-reducing effects of bright light. Furthermore, lighthas been found to influence cortisol production, but in different directions.The transition from dim to bright light in the very early morning resulted inelevated cortisol levels during the morning peak (Leproult, Colecchia,L’Hermite-Balériaux, & van Cauter, 2001). In contrast, exposure to night-time or morning bright light acutely suppressed cortisol production in analternative study (Jung et al., 2010) or showed no effect on cortisol at all(Rüger, Gordijn, Beersma, de Vries, & Daan, 2005).

SummaryBoth exposure to nature and bright (day)light can reduce stress albeit that theevidence for stress-reducing effects of daylight is still very preliminary andcertainly needs more and closer inspection. The stress-reducing potential ofnature is often studied by contrasting effects of nature with the effects ofurban environments. Moreover, not all urban environments used are equaland the attractiveness of these environments, in particular, varies substan-tially. Without a no-stimulus, or other type of control condition—such as forinstance neutral content with geometrical patterns as used by Fredricksonand Levenson (1998)—such studies do not rule out the alternative explana-tion that the effects are due to detrimental effects of urban environmentsrather than beneficial effects of natural environments. Another option istaking baseline measures to better understand the direction of effects, as wasthe case, for instance, in the study by Hartig and colleagues (2003).

Despite the issues raised above, there are commonalities in the stress-reducing effects of daylight and nature through the cardiovascular system,and in particular through activation of the parasympathetic system. More-over, both nature and light influence cortisol production, a hormone relatedto stress, although mixed results were found for the effects of light on cortisol.These differences have been attributed to differences in research designs,including duration of exposure, timing of exposure (differences in circadianphase), and intensity of the light (Jung et al., 2010). Moreover, differences inspectral composition of the light could also be a factor here. In the nextsection, we will take a closer look at a concept closely related to stress, namelymood.

MOOD

An important outcome of stress is a change in mood, in particular an increasein negative affect and a decrease in positive affect. Several scholars have



proposed a relation between positive mood and physical health (e.g. seeSeligman & Csikszentmihalyi, 2000). In an extensive review on how positiveaffect influences health, Pressman and Cohen (2005) found convincing evi-dence for beneficial effects of positive emotions on mortality, morbidity,disease severity, and subjective health. However, they also found that arousalplays an important role. When positive emotions are accompanied by arousalthey may also harm health (Pressman & Cohen, 2005). Extreme positiveemotions can have the same effect on our physiological system as the stressresponse, although the magnitude of effects of strong positive emotionalarousal on the cardiovascular system is often smaller than for negative onessuch as anger (Pressman & Cohen, 2005), and heart rate responses have beenfound to persist longer after negative than after positive emotions (Brosschot& Thayer, 2003). Therefore, calm positive emotions are especially beneficialto health (Pressman & Cohen, 2005).

Positive emotions can also improve health by helping people buildresources to buffer future stressful events. According to the Broaden-and-build theory (Fredrickson & Joiner, 2002), positive emotions broaden ourmindset, enabling more global, creative thinking. This, in turn, enables us toexpand both our mental and social resources. These resources will help uscope with future stressors more effectively, buffering potential harmful effectsof stress on our health.

The present section discusses research investigating nature and daylight’spotential to induce (calm) positive emotions and mood.

Nature and MoodAccording to psycho-evolutionary theory (Ulrich et al., 1991), unthreateningnatural environments should evoke a pre-cognitive affective response. Auto-matic affective responses to natural environments have been reported as well(Hietanen & Korpela, 2004). However, note that these rapid affectiveresponses to natural versus urban environments were not replicated in a recentstudy (Beute & de Kort, 2013).

As discussed in the previous section, viewing unthreatening nature afterstress can result in an increase in positive affect and a decrease in negativeaffect (Berman et al., 2008; Hartig et al., 2003; Ulrich et al., 1991). Hartig andcolleagues (2003) compared mood effects of walking in a natural environ-ment to walking in an urban environment. They found that positive affectincreased during the walk in the natural environment, and decreased in theurban environment. Conversely, anger and aggression decreased in thenatural environment but increased in the urban environment. In the study byUlrich and colleagues (1991) described earlier, participants viewed a video ofeither natural environments or urban environments after watching a fear-inducing movie. They found that participants who watched the nature video



reported lower levels of fear and anger/aggression and higher levels of posi-tive affect than participants watching urban settings.

Daylight and MoodThrough evolution, we have developed a diurnal rhythm of being activeduring the day and resting during the night. Daylight, therefore, has evolu-tionary relevance, and Ulrich (2008) argues that under certain circumstancespositive responses to nature may be enhanced by daylight. People do indeedprefer sunny and light environments to overcast and dark environments(Beute & de Kort, 2013).

The mood-enhancing effects of daylight exposure have been investigatedin a number of studies. Exposure to daylight for 30 minutes (appr. 3,000lux falling on the eye) was found to improve positive mood in comparisonto modest levels of electric lighting (lower than 100 lux on the eye and onthe desk), although this was not accompanied by a decrease in fatigue orsadness (Kaida, Takahashi, & Otsuka, 2007). Similar mood effects ofhigher light levels (> 1,000 lux) were reported in a field study, in whichmildly seasonal participants wore a wrist-mounted device registeringlight exposure and reported on social interaction and mood (aan hetRot, Moskowitz, & Young, 2008b). Technically, the device cannotdistinguish between electric and natural light sources, but light levels over1,000 lux very likely represent daylight exposure. Furthermore, mood ingeneral was also better in summer than in winter. No relation was found,however, between an aggregate (not momentary) daily light dosage andpleasure and arousal in the general population (Hubalek, Brink, & Schierz,2010).

The amount of daylight and sunshine are of course closely related tothe weather. Interestingly, weather type is often used as an externalinformation source for our current mood and vice versa (Messner &Wänke, 2011; Schwarz & Clore, 1983). Studies investigating the effects ofweather on mood have, however, found mixed results. More sunshine hasbeen found to be related to decreased negative affect and tiredness in anonline diary study (Denissen, Butalid, Penke, & van Aken, 2008). In anexperience sampling study comparing momentary assessment of mood withhourly weather data, a relation was found between positive affect andamount of sunlight, while darkness was inversely related to alertness(Kööts, Realo, & Allik, 2011). To complicate this picture, effects of weatheron mood are dependent on age, amount of time spent outdoors, the season,and personality type (Denissen et al., 2008; Keller et al., 2005; Kööts et al.,2011).

As we reported earlier, sunlight on the skin promotes the synthesis ofvitamin D, which has been linked to the production of serotonin



(Landsdowne & Provost, 1998). Serotonin, among other functions, is a neu-rotransmitter related to mood, suggesting that via this route sunshine canalso improve our mood. Indeed, serotonin concentrations in jugular veinshave been found to be positively correlated to sunshine (Lambert, Reid,Kaye, Jennings, & Esler, 2002). Similarly, oral intake of vitamin D supple-ments during winter has been found to increase positive affect(Landsdowne & Provost, 1998). Besides effects on serotonin production bysunlight touching the skin, it has also been proposed that retinal exposureto bright light can increase brain serotonin production (aan het Rot et al.,2008a).

The mood-enhancing effects of light appear to extend beyond naturallighting. The use of bright electric light exposure has also been associatedwith positive mood effects in a field study in office environments (Partonen &Lönnqvist, 2000). Office workers received four weeks of additional brightlight exposure at their desk (appr. 2,500 lux, 6,500 K), alternating with fourweeks of no additional bright light exposure. Bright light exposure reduceddepressive symptoms and increased feelings of vitality. Note that this studywas conducted in Finland, in the darker months of the year, with highmorbidity rates for seasonal affective disorder. An improvement in mood wasalso found after bright light exposure at the workplace for people withsubsyndromal symptoms of Seasonal Affective Disorder (Avery, Kizer,Bolte, & Hellekson, 2001). Although neither study focused on persons diag-nosed with SAD, it is possible that the mood effects found are due to bettersynchronisation of the biological clock.

On a different note, brain research is indicating that light—depending onits spectral composition—directly modulates the processing of emotionalstimuli, with blue light increasing responses to auditory emotional stimuli(Vandewalle et al., 2010). The authors project that their findings may helpunderstand the mechanisms by which changes in lighting environmentimprove mood in mood disorders as well as in the general population (seealso the later section on mental health). Recall that daylight abundantlycontains this blue component, whereas electric light—depending on the spe-cific light source—generally contains much less.

Effects of light may even exist on a conceptual level. Light and darkare often used in our daily lives as a metaphor for the good versus thebad. Social psychology studies suggest that when making evaluations,people automatically assume that bright objects are good whereas darkobjects are bad (Meier & Robinson, 2004). Furthermore, people generallyassociate light with positive affect and dark with negative affect (Meier,Robinson, Crawford, & Ahlvers, 2007). Interestingly though, later studiesby Lakens, Semin, and Foroni (2012) indicated that the positive conceptualassociation of brightness is only activated when it is contrasted withdarkness.



SummaryThe literature reports that both daylight and nature can positively influencemood. Mood, in turn, can have health protective effects especially when calmpositive emotions are elicited. This section demonstrated that—as opposed toresearch into stress—a strong empirical basis exists for the beneficial effects ofdaylight on mood.

Again, most studies concerning the effects of nature on mood used aparadigm in which effects of nature were compared to effects of urbanenvironments, leaving some ambiguity about whether effects are due to thepositive influence of nature or to a detrimental effect of urban environmentsalbeit that again some studies have used a baseline comparison. For daylight,most of the research has focused on biological mechanisms underlying effectsof daylight on mood and in some cases electric bright light exposure indeeddid show similar effects to daylight exposure. Sometimes the type of source(natural vs. electric light) and the light intensity (illuminance on the eye) wereconfounded in research designs.

In the next section, we will take a closer look at a third way through whichnature and daylight can be good for health, namely by affecting self-regulation and executive functioning.

SELF-REGULATION AND EXECUTIVE FUNCTIONING

The lion’s share of behaviors that we engage in are automatic and requirelittle conscious thought. When we overrule our inclinations or feelings, weneed to exert (conscious) control over thoughts and behaviors. This processhas been labeled self-control or self-regulation. Executive functioning, ahigher-order cognitive process (Suchy, 2009), is often mentioned as theprocess through which self-regulation is exerted. Associations have beenfound between executive functioning and several diseases (Williams &Thayer, 2009), stressing the importance of having high self-control. Self-regulation, however, does not come without a cost. Executive functioningrequires much energy (Suchy, 2009) and has been found to rely on a limitedresource (Baumeister, 1998; Kaplan & Berman, 2010). Depletion of thisresource has been labeled ego-depletion within the self-control strengthmodel (Baumeister, 1998) and may be very closely related to directed atten-tion fatigue as referred to in Attention Restoration Theory (Kaplan &Berman, 2010).

A term related to ego-depletion is subjective vitality, or “one’s consciousexperience as possessing energy and aliveness” which can further be definedas “having positive energy available to or within the regulatory control ofone’s self” (Ryan & Frederick, 1997, p. 530). Subjective vitality is related toboth physical and psychological well-being and is a complex and dynamic



concept, influenced by both somatic and psychological factors. Sleep pat-terns, blood glucose level, diet, exercise, social relatedness, mood, and thesatisfaction of basic psychological needs can all influence subjective vitality(Ryan & Frederick, 1997). Ego-depletion can be overcome by increasingvitality, and people who feel vital will replenish their resources faster(Muraven, Gagné, & Rosman, 2008). In other words, factors that increasevitality will also help overcome ego-depletion.

Because of the limited capacity for self-control and its implications forhealth, finding ways to overcome depletion—ego-replenishment—can yieldpositive health outcomes. Earlier research has shown a number of waysthrough which ego-replenishment can occur, for instance consuming glucose(Gaillot et al., 2007), positive affect (Tice, Baumeister, Shmueli, & Muraven,2007), and autonomy (Muraven et al., 2008). If nature and/or daylight couldalso serve this goal, this would provide individuals with a free and generallyavailable source for recovery. Indeed, there are indications in the literaturethat this could be the case, as discussed in the next section.

Nature and Executive Functioning/Self-RegulationEgo-depletion is typically demonstrated by having participants perform atask that requires self-control (and a control group performing a task thatdoes not rely on this resource) and testing performance on a subsequent task,which also requires self-control. Ego-replenishment can be studied by addinga manipulation in between the two depleting tasks.

Recently, a study was carried out that tested nature’s effects in such anego-replenishment paradigm (Beute & de Kort, 2011). Between two ego-depleting tasks, participants were either exposed to no images, urban pic-tures, or natural pictures. In a fourth condition (the control condition)participants were not depleted in the first task. Comparing performance tothat in the control condition, Beute and de Kort report a decline in perfor-mance for participants watching urban images or no images, whereas par-ticipants who watched natural pictures performed as well as those who hadnot been depleted. These results suggest ego-replenishing effects of nature.

As mentioned earlier, the theoretical link between ego-depletion andrestorative effects of nature has only been made recently (Kaplan & Berman,2010). Consequently, only few studies have explicitly employed this paradigmto test ego-replenishing effects of nature. However, quite a few studies reportpositive findings on executive functioning after mental fatigue induction,which also provide support for the beneficial effects of nature on self-control.A number of studies followed a paradigm of inducing attention fatigue,followed by an outdoor walk and a subsequent cognitive performance test.Hartig and colleagues (2003) report that performance on an attention task(Necker Cube Pattern Control task) increased during and after walking in



a natural environment but not in an urban environment. Berman andcolleagues (2008) also found an increase in cognitive performance for thosewalking in a natural environment compared to those who walked in an urbanenvironment. In both studies, improvements in executive functioning wereaccompanied by improvements in mood. This is relevant, as Tice andcolleagues (2007) argue that positive affect may be the driving mechanismbehind ego-replenishment. However, statistical analysis indicated that inBerman’s study (Berman et al., 2008), mood did not mediate the effect ofnature on cognitive performance. Other studies were conducted in a labora-tory and involved participants watching pictures of nature. These studies alsodemonstrated that performance on tasks requiring directed attentionimproved after watching pictures of natural environments, but not of urbanenvironments (Berman et al., 2008; Laumann et al., 2003). In addition,Laumann and colleagues (2003) found that watching a nature videodecreased attentional selectivity. Kuo and Sullivan (2001) found that resi-dents who had more greenery surrounding their homes performed better onan attention task (the Backwards Digit Span Task) and displayed less aggres-sive behavior. In general, there is quite a body of experimental researchsupporting the idea that nature helps recover from attention fatigue.

In addition to experimental explorations, cross-sectional and quasi-experimental research has been employed to test the beneficial effects ofnature. Experience sampling studies produced evidence that being in naturecan increase subjective vitality (Ryan et al., 2009). More specifically, beingoutdoors increased subjective vitality and this effect was mediated by thepresence of nature. As we have seen, vitality can help overcome ego-depletion.Tennessen and Cimprich (1995) found that students living in dorms with morenatural views performed better on executive functioning tasks.

Daylight and Executive Functioning/Self-RegulationExposure to bright light—natural or electric—has been found to significantlyincrease vitality in healthy office workers (Partonen & Lönnqvist, 2000) aswell as in working-aged people with mild depressive symptoms (Leppämäki,Partonen, & Lönnqvist, 2002). In the experience sampling study by Ryan andcolleagues (2009), being outdoors increased vitality. This effect might be dueto daylight exposure, but can also be attributed to other factors (e.g. physicalactivity or exposure to nature).

Direct effects of bright diurnal light on cognitive performance—and inparticular executive functioning—have been reported. First these were estab-lished for individuals who had experienced substantial sleep deprivation(Phipps-Nelson, Redman, Dijk, & Rajaratnam, 2003). Later, similar benefi-cial effects of bright light on cognitive performance during the day were alsofound without sleep deprivation (Smolders, de Kort, & Cluitmans, 2012).



Both studies found effects of light on vigilance tasks. However, improve-ments on more complex tasks were reported in a recent study investigatingexecutive functioning after spending eight hours in mainly daylight comparedto only electric light (Münch, Linhart, Borisuit, Jaeggi, & Scartezzini, 2012).Note that since light intensities in the daylight condition were much higherthan the electric light condition, it is unclear whether in this particular studyeffects were due to the higher light intensity or were caused by other charac-teristics of daylight. An fMRI study has further demonstrated that beingexposed to bright light during cognitive performance influences thalamicactivity (Vandewalle et al., 2009). These thalamic brain structures are in turnrelated to cognitive performance. For now, it appears safe to say that vigi-lance is likely to benefit from bright light exposure, but for other elements ofexecutive functioning insufficient empirical evidence exists.

SummaryBoth daylight and nature may influence health by helping overcome or evenpreventing depletion of self-regulation resources. The evidence for nature’sattention-restoring capacity is stronger than for daylight: studies have dem-onstrated effects of nature on both objective performance and on subjectivevitality, and so far one study has demonstrated ego-replenishment by nature.Whether these effects are due to the information processing characteristics ofnature, or instead run via positive affect induced by nature remains uncertain.

The evidence for daylight effects on executive functioning and self-regulation is as yet less strong. Bright light—electric or natural—has beendemonstrated to improve vitality, a concept closely related to executive func-tioning, and to enhance performance on vigilance tasks. Evidence on morecomplex tasks requiring executive functioning or self-regulation is as yetlargely unavailable. Furthermore, the studies differ substantially in durationof exposure.

Some additional evidence for possible beneficial effects of nature and day-light on executive functioning is provided by the fact that both nature anddaylight have been found to influence heart rate variability (cf. Beute & deKort, 2013, and Rechlin et al., 1995) which, in turn, has been found to berelated to self-regulation strength (Segerstrom & Nes, 2007). In the nextsection, we will further discuss the effects of nature and daylight on physicalhealth.

PHYSICAL AND MENTAL HEALTH

Nature and HealthEpidemiological research has illustrated a positive association betweenamount of green space in the proximity of the home and health (Maas,



Verheij, Groenewegen, de Vries, & Spreeuwenberg, 2006), between theamount of accessible green space and longevity (Takano, Nakamura, &Watanabe, 2002), and between the amount of urban green space and mentalhealth and well-being (White, Alcock, Wheeler, & Depledge, 2013). Maasand colleagues found that people with a higher percentage of green space inthe proximity of their home reported better general health. Effects of viewcontent on reported health have been found in an office setting as well. Peoplewith a more natural view at work reported better subjective health (R.Kaplan, 1993). Moore (1981) further found that inmates with a more naturalview visited the doctor less often than those overlooking a courtyard. Physi-cal exercise in natural environments has also been found to be more beneficialthan in other environments (e.g. see Thompson Coon et al., 2011). Beingphysically active is often seen as a secondary benefit of nature exposure(Ward Thompson & Aspinall, 2011).

In a clinical context, Ulrich (1984) found an effect of view content onrecovery after surgery. He found that patients with a natural view had ashorter length of stay, received less negative notes from the nurses, andrequired less pain medication than patients overlooking a brick wall. Anurban upbringing has further been related to increased risk of developingschizophrenia (van Os, Kenis, & Rutten, 2010). Similarly, Ellett, Freeman,and Garety (2008) found that walking in busy urban environments resulted inincreased mental health problems for participants with persecutory delu-sions. Roe and Aspinall (2011) conducted a study comparing restorativeeffects of walking in a natural environment between healthy participants andparticipants with mental health issues. They found that walking in a naturalenvironment resulted in positive changes in mood and in mindset in relationto personal projects for both groups, but beneficial effects were greater forpeople with mental health problems compared to healthy individuals. Asecond study, investigating the effects of natural environments compared tourban environments, revealed similar results for both groups. However, theyalso found a small restorative effect of walking in an urban setting for theindividuals with mental health problems, indicating that for them the mereeffect of physical activity or exposure to daylight may have exhibited benefi-cial effects as well.

Three studies have examined the effects of natural surroundings on chil-dren with ADHD. Taylor and Kuo (2009) found beneficial effects of walkingin natural settings as opposed to urban settings on concentration of childrenwith ADHD, and van den Berg and van den Berg (2010) also reportedbeneficial effects of nature on concentration. Furthermore, playing in greensettings was related to fewer symptoms (Taylor & Kuo, 2011). Hartig,Catalano, and Ong (2007) found that the use of antidepressants in Swedenincreased when the weather in summer was bad. They attribute this effect tothe lack of restoration potential, but it could also be due to a lack of daylight.



Further effects of daylight on health will be discussed in the next section. Insum, although the majority of research in this domain has been correlational,quite extensive empirical work describes a relation between nature andhealth.

Daylight and HealthSunlight can have protective as well as detrimental effects on health. Too littleexposure to sunlight can be detrimental (e.g. Seasonal Affective Disorder,rickets; vitamin D deficiencies), but too much exposure can be harmful as well(e.g. skin cancer). The beneficial effects of sunlight have often been ascribedto the production of Vitamin D, also called the sunshine hormone or“soltriol”. Vitamin D production has been found to exhibit health protectiveeffects on a number of diseases as for instance depression, cancer, cardiovas-cular disease, influenza, diabetes, and some autoimmune diseases (Kauffman,2009). Furthermore, a relation between vitamin D and mental health has alsobeen established as neonatal vitamin D deficiency has been linked to anincreased risk of schizophrenia (McGrath, Burne, Féron, Mackay-Sim, &Eyles, 2010). Research by Krause, Bühring, Hopfenmüller, Holick, andSharma (1998) has indicated that sunbathing with UVB radiation can helpovercome mild hypertension. Sunbathing with UVA radiation, however, wasineffective in lowering blood pressure as only UVB radiation resulted in anincrease in vitamin D production.

Two studies have reported effects of daylight on recovery from physicalillness. Beauchemin and Hays (1998) have studied the effects of daylight onlength of stay and mortality rate in a cardiac intensive care unit by comparingunits located on the south side (sunny) with units located on the north side(dim). They found that women had a shorter length of stay in sunny rooms,whereas no differences in length of stay were found for men. Moreover,overall mortality rates were higher in the dim rooms than in the bright rooms.The authors postulate that depression may be one of the mechanisms throughwhich light can affect length of stay and mortality rates, since depression canhave negative effects on cardiac outcomes. A second study investigated theeffects of daylight entrance on both health and psychological outcomes forpatients recovering from spinal surgery (Walch et al., 2005). The amount ofpain medication used was monitored as well as psychological well-being,including perceived stress, anxiety, and subjective pain perception. Dimpatient rooms were located on the east side, where an adjacent buildingblocked incoming sunlight, whereas bright rooms were located on the westside. Patients in the bright rooms used significantly less pain medicationduring the first day after surgery, reported significantly less stress at dis-charge, and reported a marginally greater decrease in pain than patients inthe dim rooms.



Epidemiological studies have further revealed differences in the occurrenceand severity of several illnesses between different latitudes and in differentseasons, as for instance certain types of cancer and cardiovascular disease(Freedman, Dosemeci, & McGlynn, 2002; Kauffman, 2009; Wallis,Penckofer, & Sizemore, 2008).

Effects of light on the circadian rhythm can result in both positive andnegative health outcomes. Negative effects often pertain to the shifting in—ordisruption of—the circadian rhythm by exposure to light at the wrong bio-logical time. These detrimental effects are mostly caused by electric light asdaylight is usually only available at the right biological time. However,differences in photoperiod between seasons can affect mental health.Through light exposure, the circadian rhythm of our body is synchronisedwith the light–dark cycle of our environment. Several circadian rhythms areorchestrated by the biological clock, as for instance the release of the hor-mones cortisol and melatonin, but our cardiovascular system and core bodytemperature also follow a diurnal rhythm (Rüger & Scheer, 2009). Moreover,light exposure has been related to sleep quality through the biological clock(Hubalek et al., 2010; Riemersma-van der Lek et al., 2008). Conversely, dis-eases to the eye reducing retinal phototransduction have been linked withsleep disorders (Schmoll, Lascaratos, Dhillon, Skene, & Riha, 2011).

There are strong indications that a shortage of light is in effect at leastpartially responsible for the emergence of a condition labeled Seasonal Affec-tive Disorder (SAD; Rosenthal et al., 1984). Even in the general population,negative effects such as sadness, irritability, anxiety, lethargy, increased appe-tite, carbohydrate craving, and hypersomnia (Rosenthal et al., 1984) areexperienced especially during winter and can vary in intensity from none ormild to debilitating (Schlager, Schwartz, & Bromet, 1993).

Among the most recommended and effective treatments for SAD today isoptimally timed exposure to bright light (Terman & Terman, 2005). Currentbright light therapy almost exclusively uses electric bright light. However,depending on latitude, the prevailing daylight during winter may be equallyefficient, even on overcast days (Wirz-Justice et al., 1996). Wirz-Justice andcolleagues (1996) found that a one-hour walk per day outdoors also resultedin a decrease in symptoms. This finding is particularly interesting since somehave attributed SAD to grey cloudy weather as well (aan het Rot et al.,2008b). Unfortunately, Wirz-Justice’s study could not rule out that the effectswere (partly) due to physical exercise per se. More importantly, the effects ofoutdoor scenery also cannot be excluded.

The effectiveness of bright light therapy for non-seasonal forms of depres-sion has also been established. A very recent study demonstrated that brightlight therapy improved mood and enhanced sleep efficiency in elderly patientswith major depressive disorder (Lieverse et al., 2011). In a clinical setting,beneficial effects of bright versus dim hospital rooms have also been reported



on depression. Two studies reported a relation between length of stay and theamount of sunlight entering the patient room. Beauchemin and Hays (1996)found that patients diagnosed with severe depression had a shorter length ofstay in bright rooms than patients admitted to dim rooms. Benedetti,Colombo, Barbini, Campori, and Smeraldi (2001) found a shorter length ofstay for bipolar patients in rooms receiving sun in the morning (facing east)than patients in rooms receiving sunshine in the evening (facing west). Theyonly found this difference in summer and fall and only for patients withbipolar depression (not for unipolar depression). Adding bright light therapyto exercise programs can enhance the decrease in depressive symptoms(Leppämäki et al., 2002).

Besides seasonal and non-seasonal depression, light therapy has also beensuccessfully used to treat a range of other disorders including premenstrualdysphoric disorder, bulimia nervosa, dementia, and Parkinson’s disease(Wirz-Justice, Benedetti, & Terman, 2009). A formal link between the preva-lence of ADHD and solar intensity has recently been reported as well, withhigher prevalence in areas with lower solar intensity (Arns, van der Heijden,Arnold, & Kenemans, 2013).

SummaryNature and daylight can influence both mental and physical health on manyoutcomes including depression, longevity, and general health. This sectionhas again indicated the large overlap in beneficial effects found on health. Forinstance, both vitamin D deficiency and living in urban areas have beenlinked with an increased risk for schizophrenia, both the amount of daylightentrance and nature viewed through windows in a clinical setting have beenfound to influence recovery after surgery, and both bright light therapy andnatural environments were found to increase beneficial effects of physicalexercise on mental health.

In general, there appear to be more studies reporting on direct effects onhealth in light research than for natural views, particularly in the clinicaldomain. Chronotherapy—a combination of light exposure and sleep–wakerestrictions—has been used to treat a multitude of disorders, both concern-ing mental health (e.g. depression) and physical health (e.g. rheumaticarthritis). The exact underlying pathway(s) of the beneficial effects of lightare not clear yet. Whether the effects of light in the health interventionsreported here should be attributed to the amount of direct sunlight, illumi-nance at the eye, photoperiod, or to biological versus psychological path-ways remains largely unknown. A multitude of different daylight indiceshave been used to study its effects on health, as for instance horizontalirradiance, intensity, or spectral composition. Furthermore, it is alsounknown whether biological effects are due to acute alerting and affective



responses, or to its effects on circadian rhythm and sleep quality. Epidemio-logical studies have investigated both effects of exposure to sunlight andamount of green in the proximity. None of these studies, however, have usedcombined data sets to simultaneously investigate effects of sunlight andnature exposure on health, while in at least some studies an interaction ofthese two phenomena can be expected.

DISCUSSION: EFFECTS OF NATURE AND DAYLIGHTON HEALTH

We have presented an overview of how both nature and daylight can posi-tively influence health in a variety of ways. Although not providing a system-atic review in the strict sense of the word, our aim was to provide an overviewof the range of classes of health effects and mechanisms and to bring togetherempirical findings from different research fields. Results were reported notonly from different research fields, but also on different levels of analysis.Effects were found on an individual level (e.g. Hartig et al., 2003; Kaida et al.,2007) as well as the community level (e.g. Maas et al., 2006; Freedman et al.,2002). We have discussed effects on health directly, but also concepts closelyrelated to health: stress, mood, and executive functioning and self-regulation.These phenomena do not exist in isolation but are all closely intertwined. Forinstance, stress can cause changes in mood, and executive functioning hasalso been related to stress (Williams, Suchy, & Rau, 2009).

Commonalities have been found, in particular, in the effects of both phe-nomena on mood, activation of the parasympathetic nervous system, (lowerlevel) self-regulation, recovery and mental health. Higher light levels havebeen associated with better mood (Kaida et al., 2007; Kööts et al., 2011;Partonen & Lönnqvist, 2000; aan het Rot et al., 2008b), as have naturalenvironments (Ulrich et al., 1991; Fredrickson & Levenson, 1998; Hartiget al., 2003). Furthermore, both bright light and natural environments havebeen found to increase vitality (Ryan et al., 2009; Partonen & Lönnqvist,2000). Natural environments speed up cardiovascular recovery after stressinduction (Fredrickson & Levenson, 1998; Laumann et al., 2003; Ulrichet al., 1991), whereas exposure to bright light appears to affect the cardio-vascular system by improving heart rate variability (Rechlin et al., 1995).Furthermore, our cardiovascular system is highly dependent on our circadianrhythm. Viewing, or being in, a natural environment has been found toimprove executive functioning (Berman et al., 2008; Hartig et al., 2003;Laumann et al., 2003) and first indications for effects of bright light onexecutive functioning have been found as well (Phipps-Nelson et al., 2003;Smolders et al., 2012). In a clinical setting, both natural views and moredaylight entry affect recovery (Ulrich, 1984; Beauchemin & Hays, 1998;Walch et al., 2005). Lastly, both an urban upbringing and lack of vitamin D



during childhood were associated with an increased risk of developing schizo-phrenia (van Os et al., 2010; McGrath et al., 2010).

The substantial overlap in effects indicates a risk of focusing on only one ofthe two phenomena, because an effect of the other phenomenon cannotalways be ruled out (van den Berg, 2005). For instance, in the study by Walchand colleagues (2005) investigating the effects of light exposure on patientrecovery, dim rooms received less daylight because of an adjacent building,which may have also blocked the view of patients in these rooms. Similarly, inthe study by Ulrich (1984), the patient rooms overlooking a brick wall prob-ably received less daylight than the rooms overlooking nature. The samepertains to the epidemiological studies on nearby nature, as enjoying natureoutdoors is likely to go hand in hand with daylight exposure, and betterweather might also increase the propensity to visit nature outdoors, as thestudy by Hartig and colleagues (2007) illustrates. These examples indicate thecomplexity and interrelatedness of both phenomena. Fortunately, many ofthe studies reported here have controlled for possible confounds to a certaindegree, although not always deliberately. For instance, laboratory studiesusing pictures or videos of nature often rule out any effect of direct exposureto daylight and some studies have even reported keeping the weather condi-tions on the visual content the same. Furthermore, in some field studiesparticipants in both the nature and the urban conditions walked outside in thesame weather conditions. Similarly, laboratory experiments investigating theeffects of bright light exposure have often ruled out the effects of view content.

In future studies, we would recommend explicitly controlling for possibleconfounds of one phenomenon, when investigating the other, and to includea more detailed description of how the authors have controlled for potentialconfounds. The latter would simultaneously aid the design and performanceof replication studies (Annerstedt & Währborg, 2011; Veitch & Galasiu,2012). For nature studies, this could include weather type, time of day of theexperiment or when visual stimuli were collected, and a description of thelighting situation. For daylight studies, natural elements in the proximity andview content should be described. In addition, it has been argued that there isa need for a more detailed report of the lighting conditions in lightingresearch as well. Currently, most studies in the lighting domain report light-ing conditions in terms of the visual system, corrected for the spectral sensi-tivity of the cone receptor system (expressed in the Vλ curve). Importantly,though, this curve does not adequately represent the action spectrum of thenon-visual (ipRGC) system. There is a strong need for a more extendeddescription of subjects and lighting conditions, including age and visual stateof the subjects and spectral composition and/or irradiance measured at eyelevel (CIE, 2009; Veitch & Galasiu, 2012).

A challenge in comparing the effects of daylight and views of nature thatwe have encountered is that they are studied in separate research domains,



resulting in the use of different research paradigms and different outcomevariables. For research into the effects of view content, recovery is measuredafter induction of either stress or attention fatigue. For light research,however, effects are usually studied after mere exposure to either bright ordim light. In cross-sectional and epidemiological research, the effects of (day)light have been studied using a wide variety of predictors, including theamount of bright light (irrespective of it being natural or electric light)encountered during a certain period, amount of sunshine, latitude, or season.For nature, research usually looks at either the geographical size of thehometown (city/rural, etc.) or the amount of greenery in the environment.Light, furthermore, has been studied extensively as a therapeutic interven-tion, whereas nature does not yet seem to have reached this status.

There are some methodological issues in both fields that impede drawingthe causal inferences needed to implicate them as a therapeutic intervention.First of all, there is a need for controlled randomised trials (Annerstedt &Währborg, 2011; Veitch & Galasiu, 2012). Second, clinical trials often includea placebo condition, which can be difficult to implement in research investi-gating the effects of light and nature (Bowler, Buyung-Ali, Knight, & Pullin,2010; Veitch & Galasiu, 2012). It has been proposed that taking multiplemeasures including physiological measures can help overcome the absence ofa placebo condition (Veitch & Galasiu, 2012). Furthermore, taking baselinemeasures can also help draw causal inferences (Bowler et al., 2010). In studiestesting whether natural environments are superior to urban environments intheir health protective effects, it is not always clear whether nature has healthprotective effects or whether instead urban environments have detrimentaleffects on health. Moreover, urban environments, in particular, used inresearch vary substantially in attractiveness and content. If urban environ-ments have detrimental effects on health, we need to be able to better identifyspecifically which elements cause these effects. Furthermore, the evolutionarybasis for the beneficial effects of nature on health has recently come in forcriticism, mostly aimed at the relative lack of evidence for this claim (Joye &van den Berg, 2011). Moreover, the high level of segregation between researchon the effects of light and the effects of nature on health and well-being, andeven within the field of lighting research (Veitch & Galasiu, 2012), counters anintegrated research approach. The current plurality in light treatments differ-ing in spectral composition, intensity, time-of-exposure, and baseline lightexposure procedures makes drawing causal inferences difficult.

To be able to draw conclusions about the hypothesised overlap in effectsand underlying mechanisms of these two phenomena, more uniformity inresearch paradigms and dependent variables is needed. In both researchareas, the focus of research is also slightly different. For daylight, a solidevidence base exists for effects on mood and on physical and mental health.Less extensively studied are stress-reducing effects and effects on diurnal



executive functioning and self-regulation. For nature, most studies havefocused on stress reduction, mood enhancement, and improvement of execu-tive functioning and self-regulatory capacity. Direct effects on mental healthand physical health are studied less extensively. Therefore, a lack of evidencefor the effects of one phenomenon in one of the fields does not necessarilyimply that these effects are non-existent. Rather, it may have received lessattention from researchers. This holds, for instance, for the stress-reducingeffects of daylight.

Two elaborate psychological theories exist proposing mechanisms throughwhich nature affects health, whereas for daylight the focus has mainly beenon biological mechanisms as the biological clock and vitamin D production.For daylight, some psychological mechanisms have been hinted at, yet notheoretical basis for these effects exists as yet. Instead, if effects are found fordaylight they are often—almost automatically—attributed to non-imageforming pathways without considering psychological processes and some-times even without knowing the exact underlying biological pathway. Webelieve that herein lies the challenge for future research, as very similar effectsof both phenomena have been found on mood, (lower level) self-regulation,and the cardiovascular system. Daylight is part of our natural environment.And even though many research challenges can be identified, the studiesreported here all generally support salutogenic effects of both daylight andnature. However, in order to persuade clinicians to embrace these salutogeniceffects and to incorporate them into health interventions, there is an addi-tional need for a strong empirical basis using not only cross-sectional designsbut also randomised (placebo) controlled clinical trials. We hope this paperhas further pointed out the risk of confounds when focusing on only one ofthe two phenomena, while not controlling for the other and the need to reporthow possible confounds are controlled for. We believe the challenge that liesahead exists not only in finding out more about the underlying psychologicalmechanisms and their possible interrelatedness, but also in raising awarenessof these two natural phenomena and their salutogenic properties. Exposureto either daylight or nature need not cost a lot of effort or time. Naturalenvironments, for instance, have already been proposed as micro-restorativeexperiences (R. Kaplan, 1993). Moreover, they are both often freely andabundantly available, but their beneficial aspects could be exploited more byraising awareness in the general population, designers, and the medicaldomain alike.

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